1. Field of the Invention
The present invention relates to a method of forming ITO film, and more particularly, to a method of forming ITO film by optimized sequential sputter deposition of seed and bulk layers having different sputter process conditions, which is applicable to various display devices, and more particularly, to an organic light-emitting device needing an ultra-planarized Surface roughness.
2. Background of the Related Art
Generally, a transparent conducting oxide (hereinafter abbreviated TCO) film of a display device is used for an optoelectronic device due to its inherent attributes such as high optical transmission in a visible ray range and low resistance. Tin-doped indium oxide (hereinafter abbreviated ITO) among TCO films including In2O3, Sn-doped In2O3 (ITO), ZnO, Al-doped ZnO (AZO), and the like, which can be deposited at a relatively low temperature below 200° C. and has an excellent optical transmission, an excellent electrical conductivity, and an excellent etch attribute essential to a process of forming pixels of a display device, has been mainly used. An ITO film, which is a very important element of an optoelectronic device, is widely used in a flat panel display field including LCD, PDP, OLED, etc.
The ITO film is growed by physical vapor deposition (PVD) such as DC magnetron sputtering, RF magnetron sputtering, medium frequency magnetron sputtering, ion beam sputtering, and thermal evaporation or chemical vapor deposition (CVD) such as spray pyrolysis, spin coating, etc. Physical and chemical properties (specific electrical resistance, visible ray transmittance, Surface roughness, etc.) of the ITO film vary according to deposition method and condition.
Lately, many efforts are made to development of an organic light-emitting device (OLED) as a next generation flat panel display. The organic light-emitting device (hereinafter abbreviated OLED) is advantageous in self-luminescence, low voltage drive, fast response, wide viewing angle, and the like, thereby standing in the spotlight in the field of the next generation flat panel display panel, OLED adopts a current drive system, whereas LCD uses a voltage drive system. In case of the current drive system, electric charges are accumulated on a tip of a peak(protrusion) of an electrode surface to easily generate a spark, thereby degrading or causing damage to the device. Hence, great importance is attached to a surface property of an ITO film used as an anode of OLED as well as electrical and optical properties as the inherent attributes of the TCO film.
FIG. 1 is a cross-sectional view of an ITO film as a transparent conducting electrode used as an anode of OLED according to a related art.
Referring to FIG. 1, a surface of an ITO film formed by a method according to a related art has a Surface roughness of several tens to several hundreds nm. A spark is generated from a rough peak of the ITO film to cause damage to a pixel defined by patterning the ITO film on a substrate into pixel units, thereby forming a dark spot. And, an electric field according to a surface morphology varies so that some pixels are unable to function well. Moreover, it is highly probable that a pin hole may be formed because of a rough interface of the ITO film with hole transport layer (HTL), organic light-emitting layer (EL), and metal cathode layer successively stacked on the ITO film electrode (anode), i.e., ITO/HTL/EL/Ca/Ag arrangement configuration. In case of a high Surface roughness, the ITO film electrode (anode) is brought direct contact with the metal cathode layer to cause a short circuit. Thus, the Surface roughness of the ITO film electrode (anode) has great influence on operation and stability of OLED. Moreover, in case of LCD, a rough surface of an ITO film brings about charge carrier scattering that reduces electrical conductivity, whereby a surface structure of the ITO film is very important in being applied to TFT-LCD. Besides, the rough surface of the ITO film lowers optical transmission because of scattering loss.
Electrical and optical characteristics and stability of ITO film formed by vacuum sputtering including magnetron sputtering are superior to those of ITO film formed by thermal evaporation or CVD. Yet, the ITO film formed by vacuum sputtering including magnetron sputtering fails to have the planarized surface meeting the requirement for OLED. The Surface roughness requirements of the ITO film applicable to OLED are RMS (root mean square) roughness≦2 nm and Rp-v (peak to valley)roughness≦10 nm. Hence, in order to lower the Surface roughness of the ITO film formed by the current vacuum sputtering, the deposited ITO film is planarized by mechanical and/or chemical polishing. However, the mechanical and/or chemical polishing complicate the process of forming the ITO film for OLED. Besides, by-products such as polishing slurry and the like are generated and the residues of the polishing adhere to the surface thereof, whereby production yield is reduced and product costs are raised.
Other deposition methods skipping the polishing step after film deposition are tried to form an ultra-planarized ITO film without degradation of the electrical and optical characteristics. One of such deposition methods is mixed sputtering that a predetermined portion of DC sputtering is supplied by RF power. Another one of such deposition methods is depositing an amorphous ITO film instead of a crystalline ITO film. However, such methods are tried but to fail in forming an ITO film substantially applicable to OLED.